Fischer-Tropsch product condensing process for cold climates

ABSTRACT

A process to optimize the condensation of Fischer-Tropsch products coming from the Fischer-Tropsch reactor in a vapour phase with the use of light oil and, therefore; reducing the possibility of coating the coolers and piping with waxy material.

CROSS REFERENCE TO RELATED APPLICATIONS

Not applicable.

FEDERALLY SPONSORED RESEARCH

Not applicable.

REFERENCE TO MICROFICHE APPENDIX

Not applicable.

FIELD OF THE INVENTION

The invention relates to a process for condensing Fischer-Tropsch products produced in a plant located in a cold or arctic climate. More specifically, the invention relates to a process which utilizes a light oil stream to condense Fischer-Tropsch products.

BACKGROUND OF THE INVENTION

Many natural gas reserves, including stranded natural gas sources, are located in cold or arctic climates. Particular considerations must be given to the operation of Fischer-Tropsch processes in such climates. Following synthesis in a Fischer-Tropsch reactor, the light hydrocarbon product stream exits the Fischer-Tropsch reactor as vapor and is condensed, generally through the use of heat exchangers, prior to further separation and/or upgrading. In cold climates, the condensed hydrocarbon product could potentially form a solid or semi-solid layer within piping or the post-synthesis heat exchangers. Formation of such layers may result in product loss as well as reduction in heat exchanger efficiency and is, therefore, undesirable and preferably is minimized or eliminated.

Moreover, in cold climates, e.g. sustained temperatures of less than about 60° F., even short lived process upsets which reduce or stop process flow could result in significant solid wax deposition in process equipment, and particularly in piping.

There remains a need, therefore, for a process to reduce or eliminate solid or semi-solid formation of Fischer-Tropsch products in process equipment and piping.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a process for condensing Fischer-Tropsch products including the steps of: (1) passing FTR gasses through a conduit located within a Ficher-Tropsh reactor wherein the conduit is situated substantially vertically within the reactor, has a first open end which is situated with the reactor headspace, and which directs the FTR gasses downward and out of the FTR; (2) feeding the FTR gasses into a first scrubber; (3) withdrawing a first overhead stream from the first scrubber; (4) feeding the first overhead stream into a second scrubber wherein the second scrubber is capable of separating two liquid phases; (5) recovery from the second scrubber an organic liquid phase comprising a light hydrocarbon oil; and (6) directing a portion of the light hydrocarbon oil to the first scrubber and recycling a portion of the light hydrocarbon oil back into the second scrubber.

In some embodiments of the invention the process further includes withdrawing a second overhead stream from the second scrubber. In yet other embodiments, the second overhead stream is fed into a third scrubber wherein the third scrubber is capable of separating two liquid phases; and a second light hydrocarbon oil stream is recovered from the third scrubber. Alternative embodiments further include directing a least a portion of the second light hydrocarbon oil stream into either the first, second or third scrubber. In yet other embodiments of the invention, an aqueous liquid stream comprising FT-produced water is recovered from the third scrubber. In certain embodiments a portion of the second light hydrocarbon stream is further processed into a product stream.

Some embodiments of the invention include the additional step of recovering a heavier hydrocarbon stream from the first scrubber. Alternatively, an aqueous liquid stream comprising FT-produced water may be recovered from the second scrubber. In yet other embodiments of the invention, the first overhead stream contains substantially no hydrocarbons having a carbon number greater than 18. In yet other embodiments of the invention, the heavier hydrocarbon stream contains substantially no hydrocarbons having a carbon number less than 16.

In some embodiments of the invention wherein a third scrubber is used, a third overhead stream is removed from the third scrubber and then fed into a synthesis gas generation system. In other embodiments a second overhead stream is recovered from the second scrubber and the second overhead stream is cooled to a temperature greater than the freezing point of FT-produced water entrained in the second overhead 3 stream. In yet other embodiments, the process also includes the step of cooling the first light hydrocarbon oil stream to a temperature between 30 and 70° F. from the second scrubber prior to feeding into the first scrubber.

In some embodiments, a portion of the first light hydrocarbon stream is further processed into a product stream.

Yet other embodiments of the invention provide a process for condensing Fischer-Tropsch products including the steps of: (1) passing FTR gasses through a conduit located within a Fischer-Tropsch reactor wherein the conduit is situated substantially vertically within the reactor, has a first open end which directs the FTR gasses downward and out of the FTR; (2) injecting a light hydrocarbon oil stream into the FTR gasses; (3) feed the FTR gasses into a flush drum; and (4) withdrawing a first overhead stream consisting essentially of hydrocarbons having a carbon number of less than 16 from the flush drum.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a first embodiment of the process.

FIG. 2 is a graph comparing calculated hydrocarbon compositions in a typical FTR overhead and in the overhead of a first scrubber.

FIG. 3 is a schematic of a second embodiment of the process.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Referring first to FIG. 1, the product of a Fischer-Tropsch reactor (“FTR”) 1 passes through an internal conduit 2. Internal conduit 2 is positioned substantially vertically in the FTR 1, has a first end opening located within the gas phase headspace of the FTR 1, and connects to an exit tube 22 within a bottom portion of the FTR 1. Alternatively, exit tube 22 may be integrally molded with conduit 2. Exit tube 22 passes through the wall of FTR 1 so that the FTR gases exit FTR 1 at a level lower than the FTR 1 gas phase headspace. The FTR gases exiting FTR 1 include both gaseous Fischer-Tropsch hydrocarbon products as well as unreacted synthesis gas and is generally at a temperature between about 350° F. and about 520° F. The FTR gases are fed into a first scrubber 16 which is washed with a light oil stream 15. Light oil stream 15 contains primarily C₁₀ to C₁₈ paraffins and is at a temperature between about 30° F. and about 70° F. Heavier hydrocarbon products 21 in the FTR gases are condensed in first scrubber 16 and are removed from a bottom portion of first scrubber 16. The heavier hydrocarbons stream 21 contains primarily C₁₅₊ paraffins.

An overhead stream 5 leaves the first scrubber 16 and is fed into a second scrubber 7. Second scrubber 7. Scrubber 7 is configured to permit the separation of two liquid phases within a bottom portion of the scrubber 7. Fischer-Tropsch produced water 10 exits the bottom of scrubber 7 and a light oil stream 11 is withdrawn from a side port 23 of scrubber 7. In some embodiments of the invention, light oil stream 11 is cooled by a heat exchanger 6. In some embodiments of the process, light oil stream 11 is divided into three streams 13, 14, and 15. Light oil stream 13 may be used to wash a third scrubber 17, light oil stream 14 may be recycled to second scrubber 7 and light oil stream 15 may be used to wash first scrubber 16.

An overhead stream 20 is removed from scrubber 7 and optionally is cooled using an air cooler or other heat exchanger 4. Overhead stream 20 is fed into a third scrubber 17. Third scrubber 17 is also configured to permit the separation of two liquid phases in a bottom portion of third scrubber 17. A stream containing Fischer-Tropsch produced water may be removed from the bottom of third scrubber 17. A side stream containing light oil 9 also may be removed from third scrubber 17. An overhead stream 8 from third scrubber 17 contains primarily unreacted syngas.

An overhead stream 20 is removed from scrubber 7 and optionally is cooled using an air.

Referring to FIG. 3, the product of a Fischer-Tropsch reactor (“FTR”) passes through an internal conduit [2]. The internal conduit [2] is positioned substantially vertically in the FTR, has a first end opening located within the gas phase headspace of the FTR and connects to an exit tube [22] within a bottom portion of the FTR. Alternatively, the exit tube [22] may be integrally molded with conduit [2]. Exit tube [22] passes through the wall of FTR so that the FTR gases exit FTR at a level lower than the FTR gas phase headspace. The FTR gases exiting FTR include both gaseous Fischer-Tropsch hydrocarbon products as well as unreacted synthesis gas and is generally at a temperature between about 350° F. and 520°F.

The FTR gases [3] are feed into the first scrubber [16] which is washed with a light oil stream [15]. Light oil stream [15] is produced in the refinery section and has mainly C₉ to C₂₀ paraffins. Stream [15] is supplied at a rate and temperature needed for separating the heavy hydrocarbons of stream [3] and maintaining them in a liquid phase, stream [21]. The overhead of the scrubber [16] enters a second scrubber [7] where it is washed again with another stream [14] from a refinery. Stream [14] condenses the heavier components of stream [5] and is supplied at a rate and temperature sufficient for effective separation but not to precipitate any heavy hydrocarbon in stream [11]. This separator also condenses synthesized water [10]. The overhead of scrubber [7] enters a third scrubber [17] after being cooled and is washed with stream [13] that comes from a refinery.

Stream [13] and allows separating the heavier fraction of the hydrocarbon of stream [20] as well as water. The oil collected from scrubbers [7] and [17] can be combined and sent to a fractionator column for further processing. It is envisioned that the streams coming from the refinery streams [13, 14 and 15] could come from a fractionator or a sponge oil column (packed bed absorber).

An optimization of this design considers the elimination of scrubber [7] and performs the entire stream treatment in only two steps. 

1. A process for condensing Fischer-Tropsch products comprising the steps of: (a) passing FTR gasses through a conduit located within a Ficher-Tropsh reactor wherein the conduit is situated substantially vertically within the reactor, has a first open end which is situated with the reactor headspace, and which directs the FTR gasses downward and out of the FTR; (b) feeding the FTR gasses into a first scrubber; (c) withdrawing a first overhead stream from the first scrubber; (d) feeding the first overhead stream into a second scrubber wherein the second scrubber is capable of separating two liquid phases; (e) recovery from the second scrubber an organic liquid phase comprising a light hydrocarbon oil; and (f) directing a portion of the light hydrocarbon oil to the first scrubber and recycling a portion of the light hydrocarbon oil back into the second scrubber.
 2. The process of claim 1 further comprising the step of: withdrawing a second overhead stream from the second scrubber.
 3. The process of claim 2 further comprising the steps of: feeding the second overhead stream into a third scrubber wherein the third scrubber is capable of separating two liquid phases; and recovering from the third scrubber a second light hydrocarbon oil stream.
 4. The process of claim 3 further comprising the steps of: directing a least a portion of the second light hydrocarbon oil stream into either the first, second or third scrubber.
 5. The process of claim 1 further comprising the steps of: recovering a heavier hydrocarbon stream from the first scrubber.
 6. The process of claim 1 further comprising the step of: recovering an aqueous liquid stream comprising FT-produced water from the second scrubber.
 7. The process of claim 3 further comprising the step of: recovering an aqueous liquid stream comprising FT-produced water from the third scrubber.
 8. The process of claim 1 wherein the first overhead stream contains substantially no hydrocarbons having a carbon number greater than
 18. 9. The process of claim 5 wherein the heavier hydrocarbon stream contains substantially no hydrocarbons having a carbon number less than
 16. 10. The process of claim 3 further comprising the steps of: removing a third overhead stream from the third scrubber; and feed the third overhead stream to a synthesis gas generation system.
 11. The process of claim 2 wherein the second overhead stream is cooled to a temperature greater than the freezing point of FT-produced water entrained in the second overhead 3 stream.
 12. The process of claim 1 further comprising the steps of: cooling the first light hydrocarbon oil stream to a temperature between 30 and 70° F. from the second scrubber prior to feeding into the first scrubber.
 13. The process of claim 1 wherein a portion of the first light hydrocarbon stream is further processed into a product stream.
 14. The process of claim 3 wherein a portion of the second light hydrocarbon stream is further processed into a product stream.
 15. A process for condensing Fischer-Tropsch products comprising the steps of (a) passing FTR gasses through a conduit located within a Fischer-Tropsch reactor wherein the conduit is situated substantially vertically within the reactor, has a first open end which directs the FTR gasses downward and out of the FTR; (b) injecting a light hydrocarbon oil stream into the FTR gasses; (c) feed the FTR gasses into a flush drum; and (d) withdrawing a first overhead stream consisting essentially of hydrocarbons having a carbon number of less than 16 from the flush drum. 